The present invention relates generally to microwave waveguide converter apparatus and methods for efficiently converting microwave energy from modes having large angular mode numbers, e.g., whispering gallery modes or volume modes, to the HE.sub.1,1 mode, or to modes that can be readily converted to the HE.sub.1,1 mode or equivalent modes.
Waveguides are a form of transmission line used to transmit electromagnetic energy efficiently from one point to another. Waveguide modes are denominated to identify the distribution of the electric and magnetic fields within the waveguide. As indicated in the art, e.g., Electronics Desiqners' Handbook, 24 Edition (McGraw-Hill 1977) at page 8-36; or Marcuvitz, N., Wavequide Handbook, McGraw-Hill, pp. 72-80 (1951), specific modes are indicated by symbols such as TE.sub.m,n and TM.sub.m,n. TM indicates that the magnetic field is everywhere transverse to the axis of the transmission line, i.e., the longitudinal axis of the waveguide. TE indicates that the electric field is everywhere transverse to the axis of the waveguide. For rectangular waveguides, the subscripts m and n denote the number of half period variations of the fields occurring within the waveguide in the two transverse dimensions. For circular waveguides, the subscript m denotes the number of full-period variations of the transverse component of field in the angular direction, and is frequently referred to as the angular mode number, while the subscript n denotes the number of half-period variations of the transverse component of field in the radial direction. A circular waveguide mode having no angular dependence may thus be either a TE.sub.0,n or a TM.sub.0,n mode, where n is any integer. It is noted that the HE.sub.1,1 mode, which may be regarded as the superposition of a TE and a TM mode which exist only in a corrugated wall waveguide, is also referred to in this disclosure. Both the circular guide HE.sub.1,1 mode and the rectangular HE.sub.1,1 mode have very similar gaussian-like field patterns. Hence, for purposes of the present invention, a distinction need not be made between the circular and rectangular HE.sub.1,1 modes.
The new generation of millimeter wavelength gyrotrons, having output frequencies greater than 100 GHz and output power greater than 500 kW operate in modes having large angular mode numbers. Modes having large angular mode numbers are frequently referred to in the literature as "whispering gallery" modes. This terminology is believed to be borrowed from acoustic, wave theory, and the known principle where low amplitude acoustic waves (i.e., a whisper) generated at one end of a properly designed acoustic gallery are reflected along the edges of the gallery, e.g. its walls and ceiling, to a focal point at the other end of the gallery, where such acoustic waves can be readily discerned. In a similar manner, a whispering gallery mode transfers, microwave energy through a waveguide or equivalent medium by maintaining all of the energy near the walls of the waveguide or other medium, leaving the center of the waveguide void of such energy.
Disadvantageously, a whispering gallery mode, such as the TE.sub.15,2 mode provided by the Varian 140 GHz gyrotron, does not provide for the efficient transfer of high energy microwaves through a circular waveguide, over long distances, nor does it provide a radiation pattern of the microwave energy at the termination of the waveguide that allows such high energy to be efficiently used. High energy microwaves generated by millimeter wavelength gyrotrons are frequently used for radar or plasma heating applications that require the energy to be outside of the waveguide and focused or otherwise directed to a desired target or zone. It would thus be desirable if the high microwave energies in the waveguide could be directed to the desired target or zone by simply pointing or aiming the waveguide at the desired target or zone, much as a bullet in a gun is directed to a desired target by simply pointing or aiming the barrel of a gun at the target. Unfortunately, when the walls of the waveguide terminate, the microwave energy for most transmission modes, including transmission modes having a high angular mode number, has a conical pattern with a null on axis, thereby dramatically reducing the amount of energy that is received at any particular target point located a finite distance away from the end of the waveguide. What is needed, therefore, is a converter that converts the microwave energy while still within the waveguide to a mode that allows it to efficiently propagate upon termination of the waveguide to a desired target or zone.
For almost all purposes, from radar to plasma heating, it is desirable to radiate microwave energy with a pattern that has a single main lobe, with very little power in any side lobes, and with a well defined polarization. Such a radiation can be obtained using a free space mode that has a gaussian beam pattern that is directly radiated from the termination of the waveguide. Advantageously, such a free space gaussian beam is directly radiated from a corrugated waveguide propagating energy in the HE.sub.1,1 mode. Hence, a waveguide-converter that generates the HE.sub.1,1 mode from a whispering gallery or high order volume mode is needed for the efficient application of the new generation of millimeter wavelength gyrotrons referenced above.
One technique known in the art for converting whispering gallery modes to a beam of waves is described in Vlasov et al., "Transformation of a whispering gallery mode, propagating in a circular waveguide, into a beam of waves," Radio Eng. and Electron Phys., Vol. 20, No. 10, pp. 14-17 (1975). Basically, this technique, described more fully below, utilizes a wide slot cut in the side wall of the waveguide. Because in the whispering gallery mode the energy is localized near the walls of the waveguide, and further because such energy is a rotating wave, the proper positioning of the slot allows this energy to exit the waveguide, whereupon a suitable focusing mirror is used to direct it to a desired target area.
It is noted that the desired target area of the Vlasov device could, of course, be another waveguide designed to propagate the power in a more efficient mode, such as the HE.sub.1,1 mode. Such use of the Vlasov device presupposes, of course, that the energy passing out of the slot can be converted or transferred to the HE.sub.1,1 mode in an efficient manner. Unfortunately, as indicated hereinafter, the conversion efficiency to a single mode from the whispering gallery mode using the Vlasov converter is inherently limited to no greater than about 80%. Such efficiency may not be acceptable for many applications.